Optimum standardized ileal digestible (SID) tryptophan (Trp) to lysine (Lys) ratio was estimated for 67–96 kg barrows fed low protein diets supplemented with crystalline amino acids (AA). One hundred and fifty Yorkshire × Landrace barrows, with an average initial body weight (BW) of 67.3 ± 3.2 kg, were used in a 28-day feeding trial. All dietary treatments were based on corn, wheat bran, and soybean meal and were formulated to provide 10.5% crude protein and 12.6 MJ/kg metabolizable energy. The dietary SID Lys was set at 0.61% ensuring that Lys was marginally deficient for barrows of this weight range. Graded levels of crystalline l-Trp were added to the basal diet to produce diets providing SID Trp to Lys ratios of 0.131, 0.164, 0.197, 0.230, and 0.262. There were improvements in weight gain (linear and quadratic effect P < 0.01) and feed intake (linear effect P = 0.04) with increasing dietary SID Trp to Lys ratio. Increased SID Trp to Lys ratio resulted in a decrease in the serum urea nitrogen (SUN) content (linear and quadratic effect P < 0.01). The serum concentration of Trp increased with increasing dietary SID Trp to Lys ratio (linear effect P = 0.03, quadratic effect P = 0.08). Estimates of the optimum SID Trp to Lys ratios were 0.203, 0.197, and 0.214 for weight gain, feed conversion ratio, and SUN, respectively, using a broken-line model while a quadratic model produced optimum SID Trp to Lys ratios of 0.251, 0.224, and 0.249 for the same parameters. The results of this experiment indicate that the SID Trp to Lys ratio for finishing barrows is at least 0.203, which is higher than the SID ratio of Trp to Lys currently recommended by the National Research Council (NRC, 2012).